For many applications, it is presently necessary to regulate the current through an inductive load very exactly. For instance, with clocked control of an end stage for an electrohydraulic pressure regulator of an automatic transmission, the arithmetic mean value for the current must be regulated very precisely, so that the hydraulic pressure produced can likewise be adjusted exactly. This exact adjustment is important, not least to provide for easy shifting of the automatic transmission. Such precise regulation of the arithmetic mean value of the current for inductive loads is also desirable and necessary in other fields, for instance for controlling valves in suspension system adjustment or in anti-skid braking systems in motor vehicles.
In known current regulating circuits for this purpose, the accuracy is often inadequate, or the circuitry is overly complex and expensive; typically, external calibration is necessary to adjust the desired values and compensate for temperature factors. Besides the measuring resistor, a great number of other precision resistors and precision components are needed. This makes the current regulating circuit more complex and expensive, and it can be integrated with difficulty if at all; thus, the known current regulating circuits are typically formed either as discrete components or as hybrid circuits. The required calibration to attain adequate accuracy increases the expense.
THE INVENTION
The current regulating circuit according to the invention as defined hereinafter has the advantage of making integration possible, resulting in a small component at favorable cost that moreover no longer requires external calibration. The precision of the externally connectable measuring resistor and the precision of the command value, which is prepared in analog form, assure the precision of the regulated arithmetic mean value for the current. High precision is attainable in a simple manner over a temperature range from -40.degree. C. to +110.degree. C. Because of the variable reference potential and the fact that the voltage drop is picked up directly at the measuring resistor, influences such as temperature factors and voltage drops through feed lines can be kept from affecting the measurement. Although detecting the current in the inductive load by direct pickup of the voltage drop at the measuring resistor requires increased complexity and expense, since essentially all the active components must be acted upon by the variable reference potential, nevertheless this expense is negligible when the current regulating circuit is integrated.
The end stage is suitably formed as a push-pull end stage; a second switch transistor is disposed in a free-running circuit for the load.
By using MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) as the switch transistors of the end stage, the end stage can also be integrated with the rest.
To obtain a clocked signal train for triggering the push-pull end stage, the duty cycle of which depends on the control deviation, the comparison stage has a comparator that compares the output signal train of an oscillator, which generates a delta voltage, with a signal dependent on the control deviation; the oscillator is supplied by the auxiliary voltage source. The comparator output is suitably connected to the end stage via a driver stage provided with a level converter for the sake of level adaptation. The capacitor that specifies the frequency of the oscillator can advantageously be disposed outside the integrated circuit, in the form of an external component, to enable free adjustment of the oscillator frequency.
To attain a control deviation signal to trigger the comparator in cases where the command value of the current is clocked, the comparison stage has an integrator to form the control deviation; the actual and command values, respectively, are applied to the two inputs of the integrator. The integrator input acted upon by the command value of the current is applied to the variable reference potential via a capacitor.
To adapt the level of the command current value not coupled to the variable reference potential, which value is present in the form of a signal train of variable duty cycle, this signal train is delivered to the integrator via a level converter, and the level converter is acted upon by the auxiliary voltage source. To make it possible for the current through the inductive load to be ascertained from an external control unit, the two measurement pickups of the measuring resistor are connected to a subtractor-amplifier, which is supplied with the auxiliary voltage and at the output of which a status signal that can be picked up externally is formed.
To increase the measurement accuracy, the measuring resistor has separate measuring terminals, for four-point measurement.
A simple and practical embodiment of the auxiliary voltage source provides that it has a capacitor connected to the variable reference potential and supplied via the switching path of a transistor from a supply voltage source; to control the base of the transistor, a Zener diode connected to the variable reference potential is acted upon by the supply voltage source, and the capacitor voltage produces the auxiliary voltage.